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USDA Zones, Frost Dates & Calendar FoundationsPlanning Around Heat Waves and Cold Snaps
Averages are useful, but the weather that damages crops is the extreme. A plan for heat waves and cold snaps keeps one bad week from erasing a season.
Key points
- Watch the forecast and let it override the calendar.
- Harvest, shade, mulch and water in the morning before and during heat.
- Cover, water the soil and anchor covers before a cold snap.
- Spread plantings and keep supplies on hand.
- Record each season's weather and results, and let that record guide next year's calendar.
How do you watch for extremes?
Check the National Weather Service forecast daily in the shoulder seasons and during summer, and sign up for local alerts. Look at overnight lows for cold snaps and at highs and overnight warmth for heat waves. Note that frost depends on humidity as well as temperature, as NC State Extension points out, so a clear, still night can bring frost above freezing air readings.
Keep a short checklist where you will see it, so you act early and not after damage appears.
Wind can be as damaging as cold. Strong wind dries seedlings, tears leaves and knocks over tall plants. Stake and shelter young crops, and check covers are secured before a storm.
What does heat do to crops?
Oregon State University Extension lists signs of heat stress: leaf rolling, midday wilting, dry leaf edges, bud and fruit drop, bolting, sunscald and worse blossom end rot with uneven watering. Illinois Extension adds that tomato blossoms drop most often between 75 and 95 degrees Fahrenheit and that bees are less active near 90.
Cool-season crops can turn bitter, and many plants struggle above 104 degrees.
Plan a post-event check. After any heat wave or cold snap, walk the beds, trim clearly dead growth, water as needed and wait before judging damage, since some plants recover as weather settles.
- Roll, wilt, drop, bolt, scald: signs to watch.
- Check soil moisture before and during.
- Pick sensitive crops before the peak.
What do you do before and during a heat wave?
Harvest cool-season crops such as lettuce, spinach and broccoli before the heat arrives. Delay planting and transplanting, since hot soil harms seedlings. Water deeply and in the morning, keep soil from drying out and use mulch. Oregon State suggests shade cloth or temporary shade, added late in the morning and removed late in the day.
Avoid pesticide applications in extreme heat, and always follow the label. Applicator licensing rules vary by state, and a pre-harvest interval applies to many products.
Do not fertilize heavily in a heat wave or just before a freeze. New tender growth is the most easily damaged, and local nutrient management rules apply in many states.
How do you prepare for a cold snap?
Check your crops' tolerance. A light frost can damage tender plants, while hardy crops handle colder nights. Cover tender crops with frost cloth, row cover or sheets before sundown, and anchor the edges so wind does not lift them. Water dry soil ahead of a freeze, since moist soil holds heat better.
Remove covers on sunny mornings so plants do not overheat. Harvest ripe tender crops before the cold arrives.
If you grow for market, tell customers early when an extreme event will limit supply. Records of past events help you plan crop insurance conversations with an agent, and USDA RMA and FSA offices can explain programs.
How do you adjust planting dates?
Let the forecast override the calendar. If a cold snap is coming, hold transplants in a sheltered place and delay sowing tender seed. If a heat wave is coming, delay setting out transplants and sowing cool-season seed until it passes.
For example, if your transplants are ready but a heat wave is forecast, keep them in light shade and water them regularly until the weather settles.
Practice your response once. Knowing where covers, stakes and shade cloth are stored saves precious hours when a forecast changes.
How do you build resilience over the seasons?
Spread plantings over several dates and several beds, so a single event does not hit everything. Choose varieties suited to your climate, and keep a stock of row cover, shade cloth and stakes. Healthy soil with good organic matter holds water better in heat.
The U.S. Drought Monitor and NOAA climate pages give context on dry spells and seasonal outlooks. Use them as information, not as a guarantee.
Keep soil covered. Mulch softens the swing between hot days and cool nights, and bare ground loses moisture fast in wind and heat.
What mistakes should you avoid?
Waiting to cover until after the frost is the most common. Others are watering at midday or at night, using overhead sprinklers that wet leaves, and fertilizing during a heat wave.
Keep notes on each event, what you did and what survived, and adjust your plan for next time.
Sources and further reading
Last reviewed: November 2026. Educational information, ranges only. Confirm timing with your county extension office.